1. Field of the Invention
The present invention relates to elastic bodies, pressure rollers, and image heating and fixing devices for use in electrophotographic image forming apparatuses such as a copying machine and a laser beam printer, and to manufacturing methods therefor.
2. Description of the Related Art
Recently, in image heating and fixing devices for use in electrophotographic image forming apparatuses such as a copying machine and a laser beam printer, miniaturization of the device has been progressing, and concomitant with this trend, reduction of the pressure roller diameter used for the device has also been advancing. In order to ensure a nip width during fixation in compliance with a smaller pressure roller diameter, the hardness of an elastic layer covering the periphery of a core shaft of the pressure roller tends to be lower, and for example, as disclosed in Japanese Patent Publication No. 4-77315, an elastic layer formed of a porous elastic body (sponge rubber) has been frequently used in practice.
In addition, in the heating and fixing devices, a heating and fixing device, which does not perform a standby temperature control in order to conserve electrical power by improving heating efficiency, is increasingly used. When the amount of heat generated by a heater is efficiently applied to transfer paper, the heating efficiency of this type of device can be improved, and hence, improvement of the heat insulating property of a pressure roller has been tried. For example, in Japanese Patent Laid-Open No. 5-46041, a hollow pressure roller has been proposed which is supported by a shaft and is a rotatable roller having an air layer therein.
Among these rollers described above, in order to smoothly release toner therefrom, a roller provided with a fluorinated resin tube on the periphery thereof and a roller coated with a fluorinated resin have been known.
As a method for manufacturing a sponge rubber, a method using resin microballoons has been proposed. For example, as disclosed in Japanese Patent Laid-Open Nos. 8-12888 and 5-209080, unexpanded microballoons are compounded with a rubber and are then heated, so that expansion of the resin microballoons and curing of the rubber are simultaneously performed.
In addition, as a method for solving a problem (nonuniform cells) encountered in the method described above, a method has been proposed in Japanese Patent Laid-Open No. 10-060151 in which resin microballoons expanded beforehand are compounded with a liquid rubber compound, and the liquid rubber compound is molded at a temperature not more than a melting temperature of the resin forming the microballoons so as to obtain a crosslinked rubber material.
It may be easily considered that resin balloons and a silicon rubber are used for forming a sponge elastic layer of the pressure roller, and in the case described above, as disclosed in the conventional example described above, resin microballoons expanded beforehand are suitably used.
In a conventional method for manufacturing the sponge rubber using the resin microballoons expanded beforehand, for example, unexpanded microballoons (fine hollow balls), each having an outer shell formed of a thermoplastic resin and containing isobutane or the like therein, are expanded to have a predetermined diameter and are then compounded with a rubber material, and subsequently, the rubber material is vulcanized at a temperature below a melting temperature of the resin forming the microballoons, whereby a rubber (hereinafter referred to as xe2x80x9cmicrosponge rubberxe2x80x9d in some cases) containing stable microballoons, i.e., microballoons having uniform diameters even after the vulcanization, is obtained.
However, when the microsponge rubber described above is used for forming a pressure roller, since the elastic layer (microsponge rubber) is formed of closed cells, the air permeability of the roller is low, and as a result, problems may occur in some cases in that the outside diameter of the roller changes with an increase in roller temperature while the roller is used, the processing speed thereof changes, and expansion and shrinkage of an image on a recording material may occur.
Accordingly, an object of the present invention is to provide a pressure roller comprising an elastic layer formed by curing a silicone rubber material containing at least expanded resin microballoons, in which the change in outside diameter of the roller caused by an increase in roller temperature during operation is small, and is to provide a heating and fixing device which can maintain a stable processing speed.
A roller in accordance with one aspect of the present invention comprises a supporting body, an elastic layer provided on the supporting body, in which the elastic layer comprises a silicone rubber formed by curing a silicone rubber material containing at least expanded resin microballoons. The roller described above has a gas permeability indicating the recovery characteristic where, when the roller is placed in a vacuum chamber in which atmospheric pressure is reduced to 0.001 MPa or less over 2 minutes, an increased amount of the thickness of the elastic layer after 10 minutes from the start of evacuation is two-thirds or less of the maximum increased amount of the thickness of the elastic layer during a 10 minutes period from the start of evacuation.
In the roller described above, the elastic layer preferably comprises air void portions (hereinafter referred to as xe2x80x9cmicroballoon air void portionsxe2x80x9d in some cases) formed by breaking the resin microballoons and passages which are formed by evaporation of a volatile component contained in the silicone rubber material and which connect the air void portions therebetween. A cured silicone rubber material, which contains microballoon air void portions connected to each other via passages, has a superior gas permeability.
As the volatile component which connects the microballoon air void portions therebetween, a material having a good affinity to the expanded resin microballoon and a poor affinity to the silicone rubber is preferably used, and in addition, a material is preferably used which evaporates at a temperature not less than a softening temperature or a melting temperature of the resin for forming the resin microballoons. The volatile component is preferably at least one selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, and glycerin.
In addition, the silicone rubber material may be an addition type liquid silicone rubber material, and a cured product of the addition type liquid silicone rubber material is preferably processed at a temperature of 200xc2x0 C. or more so as to break substantially every microballoon shape formed by the resin and to ideally form passages which connect the air void portions therebetween.
In accordance with another aspect of the present invention, a heating and fixing device comprises a fixing member having heating means for heating and fixing an unfixed toner image on a recording material and a pressure member for pressing the recording material against the heating means, wherein the pressure means is the roller described above.
In accordance with another aspect of the present invention, an elastic body comprises a silicone rubber formed by curing a silicon rubber material containing at least expanded resin microballoons, wherein air void portions formed by breaking the resin microballoons are connected to each other by passages. In the elastic body described above, the inside diameter of the passage connecting the air void portions therebetween is preferably in the range of ⅓ to {fraction (1/30)} of the inside diameter of the air void portion. A roller comprising an elastic layer formed of the elastic body described above on a supporting body is preferably used as a pressure roller.
In addition, in accordance with another aspect of the present invention, a method for manufacturing a silicone rubber roller comprises a step of preparing a metal shaft, a silicone rubber material, expanded resin microballoons which have outer shells composed of a thermoplastic resin and which form air void portions, and a volatile component which is evaporated to connect the air void portions therebetween; a step of compounding the silicone rubber material, the expanded resin microballoons, and the volatile component for forming a compound; a step of performing heat curing of the compound at a temperature not more than a softening temperature of the thermoplastic resin to form the cured compound on the metal shaft; and a step of heating the cured compound at a temperature not less than the softening temperature of the thermoplastic resin to break the expanded resin microballoons and to evaporate the volatile component so as to form air void portions connected to each other.
The heat insulating pressure roller according to the present invention comprises the silicone rubber formed by using the resin microballoons and is used in the heating and fixing device of the electrophotographic image forming apparatus. In the pressure roller described above, since the air voids in the elastic layer are increasingly formed into open cells, the air permeability of the roller is high, and hence, the change in outer diameter thereof is small with an increase in temperature of the roller during operation. In addition, in the heating and fixing device provided with the pressure roller of the present invention, expansion and shrinkage of an image on a recording material caused by the change in processing speed do not occur, whereby a stable image can be formed on the recording material. Furthermore, the present invention provides the method for manufacturing the roller described above and provides the elastic body having a small volume change caused by the change in temperature.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.